The long-term objective of this grant is to determine how the nervous system controls goal-directed movements of the limbs. In goal-directed movements, and important task of the nervous system is to specify target location in the motor command. Target location might be accurately specified in terms of the joint torques that are needed to control for inertial forces and other mechanical consequences of limb movement. Alternatively, target location might be initially specified by approximation, based on past experience with similar motor tasks, and later specified by error correction mechanisms. In the previous grant cycle, we studied the production of torque at a single isometric joint, a motor task having minimally complex mechanical consequences. During rapid and accurate changes in torque, target torque was controlled by the sequential recruitment of three distinct control mechanisms, an initial approximation and two error correction mechanisms. One of the aims of the proposed study is to further describe these control mechanisms. Another aim is to evaluate a motor task that is biomechanically more complex, accurate movement of a single joint. We will determine if the same three mechanisms that were previously identified in torque responses, also control limb movements. Once we understand how the normal nervous system controls movement and torque production, we will be better able to understand the basis of motor system disorders. Understanding how the normal nervous system has solved the problems of limb control may also allow us to apply these solutions to similar problems in motor rehabilitation, e.g., prosthesis control and functional stimulation of paralyzed muscle. Finally, the results of our studies of young adult subjects may be compared to studies of other age groups to better understand the processes of motor system development and aging.